This invention relates to a manhole with a locked and sealed cover enabling access to an underground system, in particular an underground urban sanitary system.
It is known that such a manhole must be sealed in order to prevent the penetration of run-off water into the system. In addition the cover must be held firmly in its frame in order to prevent the movement of the cover during the passage of a vehicle. These two problems of sealing and holding are particularly crucial in hilly or mountainous regions due to the large potential quantities of run-off water and the high mechanical stresses caused by vehicles on an uphill or downhill road.
French Pat. 1,426,805 describes a manhole of a generally circular shape which is sealably locked and which comprises a cover, or plug, held in a frame by a locking device taking support on the lower surface of the frame and connected to a threaded stem traversing the plug from side to side, with a sealing ring being interposed between the frame and the cover. In this device the threaded stem which ensures the application of the locking device on the lower surface of the frame is situated at a distance from the center of the cover which is less than the radius of the sealing ring interposed between the frame and the cover. In such a device there is thus a problem of sealing the passage of the threaded stem through the cover, which cannot be resolved by a toric joint due to the stresses received by the cover during the passage of vehicles.
In addition, the locking of the cover on its frame is provided by direct contact, metal against metal, between the frame and the cover. However, in view of the foundry manufacturing tolerances, perfect contact over the entire periphery of the cover is difficult to provide. Thus, the inevitable results are movements between the cover and the frame which, on the one hand, create unacceptable noises when a vehicle passes over the manhole and, on the other hand, cause the shearing of the sealing ring.
U.S. Pat. No. 4,203,686 is also known which describes an obturation device comprising a cover held in a frame by a locking ring applied on the upper surface of the cover and which has support surfaces opposite corresponding surfaces of the frame which cooperate therewith to apply the cover on the frame when the locking ring is rotated. A sealing ring is interposed between the frame and the cover. With such a device the locking of the cover is achieved without any perforations of the cover and there is therefore no additional sealing problem. However, since the locking is simply done by rotating the locking ring and is maintained by friction, the repeated passage of vehicles can cause the inverse rotation of the locking ring, which unlocks the cover. The friction coefficient between the metallic components is such that the forces caused by such friction and opposing the rotation of the locking ring are negligible in relation to the forces caused by the passage of a vehicular load on the manhole.